The so-called air bag system in which a nylon bag stowed in the steering wheel assembly or dashboard of a car is inflated on sensing a car crash occurring at high speed to thereby protect the driver and other passengers against being injured or killed by the impact against the steering wheel or front glass is claiming a phenomenally increasing share of the market reflecting the current rigorous requirements concerning driving safety.
In this air bag system, a gas generating composition is ignited, either electrically or mechanically, in an instant on sensing a car crash to thereby inflate the bag with the gas so generated. The gas generating composition is generally supplied as molded into a pellet or disk form. It is essential that such a gas generating composition insures an appropriate burning velocity. If the burning velocity is too low, the bag cannot be inflated in an instant so that the system fails to achieve its object. The gas generating composition is a powdery composition having the property to get ignited by a shock. Shock ignitability is the sensitivity of a powder to shock ignition and an excessively high shock sensitivity is undesirable from the standpoint of safety because it represents a high risk of explosion in the course of production, e.g. in the mixing stage or in the molding stage. Therefore, shock sensitivity is preferably as low as possible.
It is also necessary that the combustion temperature of the gas generating composition be not too high. This is because, to absorb the shock of a car crash to the driver or passenger and help him to escape, generally the inflated air bag then releases the internal gas to shrink but if the combustion temperature is too high, the released gas is also hot enough to cause the passenger to sustain a burn, perforate the bag to detract from its function, or burn the bag to induce a car fire.
The known air bag gas generating compositions comprise sodium azide as the gas generating base and certain additives such as an oxidizing agent e.g. metal oxides such as TiO.sub.2, MnO.sub.2, Fe.sub.2 O.sub.3, CuO, etc., nitrates such as NaNO.sub.3, KNO.sub.3, Cu(NO.sub.3).sub.2, etc., perchlorates such as KClO.sub.4, NaClO.sub.4, etc. and chlorates such as KClO.sub.3, NaClO.sub.3, etc.!, a reducing metal Zr, Mg, Al, Ti, etc.!, a cooling agent Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3, CaCO.sub.3, FeSO.sub.4, etc.!, a pH control agent iron sulfate etc.!, a mechanical performance agent MoS.sub.2, KBr, graphite, etc.! and so on.
Such sodium azide-based gas generating compositions are in common use today partly because the generated gas is nitrogen gas for the most part and partly because they have adequate burning velocities and relatively low combustion temperatures. However, sodium azide has the following disadvantages.
(1) It has a risk for causing a fire on decomposition or combustion. Thus, since a fire may be induced in the course of production (on mixing with the oxidizing agent, in the final granulation stage, etc.), it requires a rigorous safety control. PA1 (2) It produces Na on decomposition. Since Na reacts with water to generate hydrogen and become ignited to produce a toxic fume, there is considerable difficulty in treatment. PA1 (3) It reacts with the oxidizing agent to liberate toxic substances such as Na.sub.2 O and its derivatives (such as NaOH), thus calling for careful handling in the course of production. PA1 (4) It is acknowledged that the gas generated on combustion or decomposition of sodium azide is rich in nitrogen and very lean in toxic substances so that there is practically no problem. However, for added safety, a further reduction in the level of toxic substances is desirable. PA1 (5) Sodium azide in gas generating compositions is hygroscopic and since the absorption of moisture leads to a decrease in combustibility, there must be an effective provision for the prevention of moisture absorption. PA1 (6) Since it is a toxic and hazardous substance, an additional capital investment is needed for securing safety. PA1 (a) The composition of this invention is remarkably low in toxicity and the potential to cause a fire on decomposition or combustion. Therefore, the risk of hazards in handling in the course of production is very low. It can be easily molded, too. PA1 (b) The composition of this invention has a low shock sensitivity which is either equivalent to or lower than that of the sodium azide-based gas generating composition and is, therefore, is very safe. PA1 (c) The composition of this invention is equivalent or superior to the sodium azide-based gas generating composition in burning velocity and gas output. PA1 (d) Like the sodium azide-based gas generating composition, the composition of this invention has a relatively low combustion temperature so that it does not have the risk of causing a burn to the driver, passenger or a perforation or burning of the bag. In addition, the level of toxic substances in the product gas is very low. PA1 (e) Since the base nitrogen-containing compound of the composition of this invention is not hygroscopic, it is not necessary to provide for the prevention of moisture absorption. PA1 (f) The composition of this invention can be produced at remarkably reduced cost. PA1 (g) Compared with the prior art gas generating compositions, the composition of this invention can be easily disposed of.
In view of the above disadvantages of sodium azide, the advent has been awaited of an air bag gas generating composition which, compared with the sodium azide-based gas generating composition mentioned above, would have an equivalent or lower shock ignitability, equivalent or higher burning velocity and gas output, and relatively low combustion temperature, and which is lower in the risk of fire and intoxication hazards and lower in costs than the sodium azide-based gas generating composition.
Meanwhile, several attempts have been made to use a nitrogen-containing compound as the base of a gas generating composition. For example, it has been proposed to subject a reducing metal, such as Zr or Mg, and an oxidizing compound, such as potassium perchlorate or potassium chlorate, to redox reaction to thereby ignite the gas generating base with the resultant heat of reaction. As the gas generating base, smokeless powder, nitrocellulose, azodicarbonamide, aminoguanidine and thiourea have been mentioned (Japanese Examined Patent Publications No. 9734/74 and No. 21171/74). However, the burning velocity that can be obtained by the above method is insufficient for practical application to the air bag. Moreover, since the mixture of reducing metal and oxidizing compound has a very high shock sensitivity, the risk of handling hazards is high. Furthermore, the combustion temperature is also suspected to be too high.
Japanese Unexamined Patent Publication No. 118979/75 discloses an air bag gas generating composition comprising a nitrogen-containing compound such as azodicarbonamide, trihydrazinotriazine or the like and an oxidizing agent such as potassium permanganate, manganese dioxide, barium dichromate, barium peroxide or the like. However, the use of potassium permanganate as the oxidizing agent involves the problem of low heat stability (i.e., being prone to spontaneous ignition) and the use of manganese dioxide as the oxidizing agent does not insure satisfactory shock sensitivity or burning velocity, while the use of barium dichromate or barium peroxide as the oxidizing agent gives rise to toxic substances in the liberated gas.
It is an object of this invention to provide an air bag gas generating composition having a shock sensitivity either equivalent to or lower than that of the gas generating composition based on sodium azide.
Another object of this invention is to provide an air bag gas generating composition which is either equivalent to or even higher than the sodium azide-based gas generating composition in burning velocity and gas output.
A still further object of this invention is to provide an air bag gas generating composition which is free from the above-mentioned disadvantages (1) through (6) of the azide compound.
It is a further object of this invention to provide an air bag gas generating composition which is low in combustion temperature with a lower risk of fire and intoxication hazards as compared with sodium azide.